66 ANNALS NEW YORK ACADEMY OF SCIENCES 



the broad and thick callus of the inner lip, with no trace of posterior 

 tooth, and the short, oblique siphonal canal. The outer lip is often 

 thickened, and its anterior portion grows more rapidly than the posterior 

 part, making the outline of the margin strongly sinuous. 



Potamidopsis tricarinata Lamarck 

 Plate VII, figs. 7, 8 ; plate viii, figs. 7, 8 ; plate ix, figs. 7, 8. 



1804. Cerithium tricar inatmn Lamarck, Ann. du Mus. Nat. d'hlst. naturelle, 



III, 272, No. 4. 

 1824. Cerithium tricarinatum Deshayes, Desc. des coquilles foss. des environs 



de Paris, II, 325, pi. 51, figs. 1, 8. 

 1866. Cerithium tricarinatum Deshayes, Desc. des anim. sans vert. dScouverts 



dans le bassin de Paris, III, p. 12.3. 



1902. Potamides tricarinatum Cossmann, Catal. lllust. des coquilles foss. de 



rfioc. des environs de Paris, IV, p. 69. 



1903. Tympanotoiiius tricarinatum Cossmann, Paleontologia Universalis, Cent. 



I, pi. 3, figs. 1, 2. 

 1906. Potamidopsis tricarinatus Cossmann, Essais de Paleontol. Comp., VII, 

 109, pi. 11. figs. 5, 6. 

 Measurements:" Length, 45 mm.; greatest diameter, 17 mm.; apical angle, 

 20° ; sutural angle. 87°. 



The early stages of the species are described from a young individual 

 of fifteen volutions. ^ The protoconch is missing, but the youngest volu- 

 tion preserved is probably the first beyond the protoconch, as it has a 

 diameter of only .3 mm. It has an ornamentation of two equal spirals 

 without ribs, and the same ornamentation persists on the next volution. 

 Eibs appear on the third volution, which has an ornamentation exactly 

 similar to the adult of Cerithium hicarinatum. On the fourth volution 

 the lower spiral becomes stronger and the upper one weaker, a tendency 

 which increases on the next three volutions until, on the seventh and 

 eighth volutions, the upper spiral has entirely disappeared, and the orna- 

 mentation consists merely of a single strongly nodose spiral, forming a 

 projecting shoulder angle just above the suture. The ninth volution has 

 a sub-sutural spiral the nodes of which are connected with those of the 

 shoulder angle by well-developed ribs. This type of ornamentation per- 

 sists for seven volutions more, and on the fourteenth volution a fine 

 spiral is intercalated between the two already existing. Comparing this 

 young individual with an adult, it is found that the latter is more re- 

 tarded in the growth of this fine spiral than the former. Tt does not 



* The specimen measured Is the one which seems to correspond most closely with the 

 figures of the type given in the Paleontologia Universalis, centurla i, plate 3, figs. 1, 2. 



